Engineers started filling the spaceship's external tank with liquid hydrogen fuel at daybreak, in a test at the launch pad to pinpoint the nagging trouble. One of the four fuel gauges promptly failed, and two others were not working consistently.

The problem could be anywhere in the 100 feet of circuitry between the shuttle and tank, or in the fuel gauges.

Atlantis' astronauts were supposed to examine the clogged rotary joint at the international space station.

But with the shuttle mission delayed until January, NASA moved up the joint inspection and added another chore to the spacewalk after a second component in the space station's power system failed 1½ weeks ago.

Commander Peggy Whitson and Daniel Tani made history as they floated outside well before dawn: It marked the 100th spacewalk at the space station.

They quickly headed to a mechanism that is supposed to tilt the solar wings on the right side of the space station toward the sun. The component experienced circuit breaker trips Dec. 8, possibly after being hit by a piece of space junk or micrometeorite.

Whitson and Tani found no signs of impact.

Afterward, the spacewalkers moved over to the bigger solar rotary joint, which is supposed to automatically revolve 360 degrees to keep the solar wings pointed toward the sun.

The joint — also on the right side of the space station — has been used sparingly in the past three months because of vibrations and electrical current spikes.

During a spacewalk in October, Tani found steel grit inside the 10-foot-diameter joint. Even though he collected samples of the shavings, which were returned to Earth on the last shuttle flight, engineers still do not know what is grinding inside.

Whitson and Tani looked deeper into the joint Tuesday, taking off more covers and using a dentist-style mirror on a rod to peek inside. They collected more samples, using tape to dab the grit. They were asked to remove a set of suspect bearings, but no repairs were planned during the excursion.

Tani said the magnetized grit looked like marching ants as the gears moved.

"It's hilarious," he called down. "It's like it's animated, like they're alive."

Despite the seriousness of the job, the spacewalkers enjoyed some laughs with Mission Control. One flight controller, a man, wanted to know from Whitson "if this beats cooking and sewing."

"Tell him I'm going to take care of him when I get back — maybe not the way he wants," replied Whitson, the space station's first female skipper.

Three hours later, she became the new female spacewalking champion — "the queen," according to Mission Control — racking up more time in the vacauum than any other woman.

The unrelated predicaments are curtailing power generation at the space station and could delay future shuttle visits.

Atlantis' trip to deliver the European lab, Columbus, is off until Jan. 10, a date that hinges on the results of Tuesday's fueling test. The Japanese lab, Kibo, or Hope, is supposed to follow on multiple shuttle flights.

NASA has been struggling with sporadic fuel gauge problems for two years and wants to resolve them once and for all. The gauges are part of a backup safety system; they prevent the shuttle's main engines from running on an empty tank, which could be catastrophic.